ARM: OMAP2+: Remove legacy mailbox device instantiation

The legacy-style mailbox device creation is supported currently
only for OMAP3, as all other SoCs are DT-boot only. Even on this
SoC, there are no client drivers that utilize these mailbox devices.
So, clean up the legacy-style mailbox device instantiation logic.
The mailbox devices are already present and supported for the DT
boot on OMAP3.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>